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#1 2018-03-25 10:22:20
Lunar Gravity-Assist boost to TMI trajectory?
It occurred to me after reading about the Voyager probes that a Mars mission might use the Moon for a delta-V boost, thus saving some launch mass or enabling a shorter transit to Mars.
Then it occurred to me that I'm not that clever, so someone else must have thought of it and dismissed it long ago, which would be why I've never heard mention of the idea. Or else I'm not as widely read as I'd like to think!
Here's what I found; I'm sure there are some readers much better equipped mathematically.
In this StackEchange Post it gives a formula for the ideal change in delta-V based on the approach velocity relative to the body to be used (here, the Moon), spacecraft's distance from the Moon, and the Moon's mass.
I calculated the ideal approach velocity relative to the Moon would be about 1.6 km/s for a closest approach (measured from the Moon's centre) of 1800km, so 63km above the surface (Mons Huygens, highest lunar mountain is 5.5km says Wikipedia)
With these figures, a Mars spacecraft would ideally gain about 1.6km/s delta-V.
Considering the delta-V from Earth intercept to Mars intercept is around 1 km/s, I'm thinking there must be a good reason why this won't work! Or surely we would have heard more about it.
Can anyone shed some light on this please?
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#2 2018-03-25 10:37:28
- SpaceNut
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Re: Lunar Gravity-Assist boost to TMI trajectory?
Welcome to Newmars Johnx, yes slingshot manuevers are a plus when they can be used.
I do not have time right now but will be back when I do, hopefully others will reply to this topic.
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#4 2018-03-25 11:41:35
Re: Lunar Gravity-Assist boost to TMI trajectory?
After a little more searching I have found some references to the idea of using the Moon for gravity-assisted flight to Mars. So it may be feasible after all. Here are some references :
The slingshot effect, a paper by RC Johnson - see Introduction & reference (8) which I found inaccessible
Space Stack Exchange: Using Luna for gravity assist for interplanetary probes - there's a note from a user here that says: 'Excluding Nozomi, there's a common thread in these uses: The vehicles aren't going to another planet. One reason the Moon isn't used much for interplanetary missions is because things don't line up properly, and in the rare cases when they do, the opportunities are rather short-lived.'
Part of this issue must be that the Moon has to be in that point in its orbit where a flyby will boost you towards your target.
The Nozomi reference is: 'The Japanese Nozomi spacecraft also made use of a lunar flyby. Unfortunately, things went awry on the subsequent Earth flyby and then went awry again at Mars.'
'Things not lining up properly' etc may refer to differences in orbital planes, which need extra dV to correct for. The Moon's orbital plane is about 28.5 deg. from Earth's equatorial plane. But the Moon's is also close to the ecliptic, which ought to be useful.
A quick approximation using the formula using this simplified plane change formula I get that we need a dV of about 50% of the orbital velocity for a plane change of around 30 degrees. So actually that could make a quick loop around the Moon too 'expensive' to be worthwhile, unless we can do the plane change when v is at a minimum, such as at the apogee of an eccentric orbit. I seem to remember from my efforts on a Space Sim program that it works better that way. Any thoughts on that?
Last edited by JohnX (2018-03-25 12:11:58)
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